Such gas springs adjustable in length are known, for example from U.S. Pat. No. 3,656,593. They are used, inter alia, as a height adjustment member in chair columns for vertically adjustable chairs or corresponding columns for vertically adjustable tables. This is known, for example from U.S. Pat. No. 3,711,054. This adjustment of the height of chairs or seats is effected in that the valve of the gas spring is opened while the user is sitting on the chair. If he causes his entire body weight to act on the seat and thereby on the gas spring, the piston rod together with the piston is pushed into the housing, i.e. the seat lowers. If, on the other hand, he releases the load from the seat, in that he shifts part of his body weight in seated position onto his legs, the piston will be pushed out of the housing, i.e. the seat is raised. While the lowering of the seat thus takes place in an uncramped position of the user, the latter finds himself or herself in a cramped position on raising the seat. If, on the other hand, the gas spring is used as a vertical adjustment member for tables, it will be desirable as a rule to effect the upward movement of the table top, that is to say the driving out of the piston rod, in relatively pronounced cushioned manner, whereas the lowering of the table top, i.e. the pushing in of the piston rod is to be effected in relatively light manner.
From German Pat. No. 12 63 245 a hydro-pneumatic lifting unit for furniture is known, in which simply a damping of the outward movement of the piston rod is to be achieved. A constructional improvement of this lifting unit is known from German published patent application No. 34 19 364. In neither of these cases are the depicted specific problems connected with the use of gas springs adjustable in length in chair columns dealt with.